Oil and gas pipeline developed into mid later period, the content of CO2 increasedsignificantly, martensitic stainless steel for oil well pipe can be used in theenvironment. However, due to the poor weledability of martensitic stainless steel, it was not usedwidely. In recent years, some new stainless steel improve its welding and are put into use, butthere is still no martensite stainless steel used in HFW steel pipe.A good weldability and moisture resistance CO2 corrosion of stainless steel of HFW steel pipe isunder development. The martensitic stainless steel w (Cr) =11%~13%, and Ni and a small amountof Mo, C and N. Control of chemical composition equips the steel with good corrosionresistance and weldability. Welding using inert gas protection, prevent produced in the weldingprocess of Cr in the form of oxide, welding quality is good, the welded pipe has verygood appearance and dimension.

1. The material design concept1.1 The service environment of martensitic stainless steel line pipeTable 1: target properties of a new type martensitic stainless steel alloyYield strength/MPa

tensile strength/MPa

corrosion rate ofCO2/mm/a toughness/J

HardnessHV Welding

550 620 0.1 1001MPa

40 -20 fullsize, horizontal

350

No cold crack(without preheat and post weld heattreatment)

Note: * including the weld and heat affected zone.

1.2 The basic concept of new martensitic stainless steel developmentThe basic idea of this design is to equip the martensitic stainless steel line pipe withgood weld strength,good resistance and corrosion resistance welding, as shown in figure 2.In order to get the CO2 corrosion resistant performance is good, w (Cr) must be greater than orequal to 11%; low C and N content to reduce the hardness of the weld heat affected zone; addingNi to prevent -ferrite forming and strength lower of weld heat affected zone; adding Moto improve the anti SSC ability in the H2S wet CO2 environment.

2. the development of new type martensitic stainless HFW steel pipe.2.1 Chemical compositionMartensitic stainless steel for test chemical composition seen in table below.Air cooling after hot rolling base material has a large martensitic structure, production of stripannealing in AC1 line, to achieve the required strength. The strip after molding obtained 273mm * 7.8mm HFW steel pipe. CO2 gas shielded welding to prevent the formation of oxides ofCr, using online weld heat treatment to reduce the hardness of the weld.A new martensitic stainless steel chemical compositionProject wC wSiwMnwP wS wCrwNiwMowTiwNTargetvalue 0.015 0.2 0.5 0.03 0.003 11.0 3.5 1.0 0.03 0.015

Actualvalue 0.003 0.19 0.43 0.0026 0.0013 11.0 3.5 1.0 0.027 0.0067

3. corrosion resistance3.1 general corrosion performanceAt a temperature of 100, the partial pressure of H2S 0MPa and 0.01MPa in 25% NaCl solution,corrosion tests carried out on the base metal and weld. Separately, shot peening and machining onthe specimen, 720 days, the corrosion rate is lower than 0.01mm/a, no corrosion crack.3.2 the performance of SSCSSC test On line pipe, the test conditions of 25 , H2S 0.01MPa partial pressure, CO2 partialpressure of 10MPa, 5% NaCl solution, pH value of 4, applying a 100% yield strength. 720 days,no corrosion crack.3.3 cathodic protection performanceUnder cathodic protection against hydrogen embrittlement test condition (four point bending test)are as follows:A. Weld seam - the cross weld and HFW seam, not flattened;B. Keep the welded specimen surface unchanged, machined flat and polishing, retain the originalappearance of the HFW weld joint;C. Applied stress of 100% yield strength;D. The test solution is 3.5% NaCl, before the test pH value is 5.4, after the test, pH value is 5.8(-1194mV) and 6.3 (-894mV);E. The cathode protection condition is -1194mV vs SCE (-1150mV vs Ag/AgCl), -894mV vs SCE(-850mV vs Ag/AgCl);F. The test time is 90 days, with temperature of 25.Under the conditions of -1194mV vs SCE cathodic protection, no cracks appear, even no hydrogenembrittlement when there is cathodic protection.(1) new type welded martensitic stainless steel can produce thin wall high precision stainless steelline pipe, can use gas protection welding technology for HFW.(2) no preheating before welding and weld heat treatment of new type martensitic stainless steelpipe, all properties of the weld can meet the objectives of mechanical properties, have highcorrosion resistance.